My son is an obsessed Star wars fan...so we did JTA 3 times : 1st time when he was 9 yo and he was in the middle of the group age-wise and then 2 times on our last trip (2016) when he was exactly 11 : on the first session, he was by far the oldest but he enjoyed it as it was the very 1st time for him taking the role of the "more experienced veteran" since he got to fight Vador in 1st position. He gave it all his heart since he thought it would be the last time for him as we live abroad and maybe we wouldn't have the chance to come back to WDW before the age restriction.
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A few days after we entered the park a little bit after opening, we realized that maybe he could try his luck once again to fight Vador/Seventh Sister. We managed to find a spot in one of the latest sessions of the day. That time, there were several kids that looked older and were much taller than him (12 looking teenagerish) and who battled Vador before him. Yet he enjoyed very much the experience too.
After he got out I told him that I hoped he had enjoyed it very much since that time had truly been the last time (we couldn't afford another trip before 2 years) ...he looked at me and told me : and what if we come back before my 13th birthday? Technically I will still be 12, right?
So here we are 2 years after, we are going to try to get that one last fight just before his 13th birthday in our next summer trip and he probably will be the oldest that time but I don't think he'll let anything/anyone spoil his pleasure about him being older ...this is actually what he loves about Disney : a world where grown ups can behave like kids and enjoy life like kids without feeling ashamed!
As for girls during JTA, my daughter did all the sessions with him, she's 4 years younger so she did it at 5 (she was the youngest of her group by very far then) then twice at 7 and although she's not as obsessed about SW as her brother, she's keen on proving to us that her fight choreo will be sharper this year than the years before !
And as far as we experienced it in the 3 JTA we did, there was a good balance between the number of boys and the number of girls.